An ink jet head is one of embodiments of a liquid ejection apparatus, and is configured to eject ink as liquid from nozzles onto a medium such as a recording sheet.
Japanese Published Examined Patent Application No. 3036548 discloses an ink jet head having an air bubble discharge mechanism that prevents an air bubble flowing in a reservoir part of a head from further flowing into a pressure chamber (see, for example, Paragraphs 0010 through 013 and FIG. 3). The mechanism is as follows: An air bubble (B1) flowing in a flow passage reaches the vicinity of a first air bubble discharge hole (11) to be naturally discharged from the first air bubble discharge hole (11) to the outside of a head or stagnated in a relatively wide region (1a) in the vicinity of the first air bubble discharge hole (11). An air bubble (B2) which reaches a second air bubble discharge hole (12) is naturally discharged therefrom to the outside of the head or stagnated in a relatively wide region (1b) in the vicinity of the second air bubble discharge hole (12). The air bubbles stagnated in the regions (1a, 1b) are discharged to the outside of the head by air bubble sucking operation. Fins (10) are provided at every inlet of a supply passage (2) to prevent an air bubble (B3) from flowing into the supply passage (2) and to move the air bubble (B3) toward the second air bubble discharge hole (12). FIG. 12 is a plan view showing a manifold plate 6 of a prototype ink jet head 1 that was prepared by the present inventor of this application to investigate flows of ink in a plurality of common liquid chambers 3. The ink jet head 1 has passages extending from liquid supply ports 2 via the common liquid chambers 3 and pressure chambers 5 to nozzles 4. To accommodate high resolution recording requirement in recent years, the ink jet head 1 is configured such that the nozzles 4 are arrayed into multiple rows and the common liquid chamber 3 is provided for each row of the nozzles 4. Further, to achieve stabilized ink ejection (a larger capacity of the common liquid chamber 3 is preferable), while making the entire ink jet head smaller in size, the ink jet head 1 is configured to have a common passage 7 by which adjacent common liquid chambers 3 are connected to each other at the opposite side of the ink supplying ports 2.
The common passage 7 is formed symmetrically around a virtual center surface 8 between two adjacent common liquid chambers 3 to present a substantially semi-circular arc-shape (a substantially U-shape in combination with the two adjacent common liquid chambers 3). A discharge port 9 is formed at the intermediate part in the common passage 7.
FIG. 13 shows chronologically changes of ink flows, which were obtained by simulating the ink flows when supplying ink in the common liquid chambers 3 and common passages 7 in the ink jet head 1 shown in FIG. 12. In this simulation, in a state where the pressure acting on the ink supplying ports 2 was fixed, negative pressure was given to the discharge port 9, and the pressure was gradually reduced from time 0 milliseconds. FIG. 13(a) shows a state of ink at time 0 milliseconds when supplying of ink to the ink supplying ports 2 was commenced, and FIGS. 13(b), (c), (d), (e), (f), (g), (h), (i), (j) and (k) respectively show a state of ink at 3.0 milliseconds, 4.0 milliseconds, 5.0 milliseconds, 6.0 milliseconds, 7.0 milliseconds, 7.5 milliseconds, 8.0 milliseconds, 8.5 milliseconds, 9.0 milliseconds, and 12.0 milliseconds after supplying of ink to the ink supplying ports 2 was commenced. With the ink jet head 1, if negative pressure is given to the discharge port 9 with two common ink chambers 3 in an empty state, ink is supplied from the liquid supplying ports 2 to the common ink chambers 3 as shown in FIG. 13. Therefore, the interior of the common ink chambers 3 and the common passage 7 is gradually filled with ink (Refer to FIG. 13(a) to (j)).
In case of the ink jet head 1, ink that has flown through two common ink chambers 3 further flows in the extension direction of the common ink chambers 3 even if it reaches the tip end wall side 3a of the partition wall between the common ink chambers 3 (Refer to FIGS. 13(f) and (g)). Therefore, spacing is formed at the extension portion of the tip end wall side 3a in ink that has flown through the two ink common chambers 3. Since the common passage 7 is formed to be left-right symmetrical around the virtual center surface, even if left-right symmetrical flows are made close to each other along the common passage 7 and join together, air in the spacing of the extension portion of the tip end wall side 3a is left as an air bubble 10 (Refer to FIG. 13(i)). Even if the discharge port 9 is provided in the vicinity of the tip end wall side 3a, an air bubble 10 is left over at the end part of the downstream side of the common passage 7 (Refer to FIG. 13(j)). Since the flows are left-right symmetrical, joined inks flow toward the discharge port 9 as is without producing any vortex flow. Accordingly, the air bubble 10 left over subsequently stagnates as it is (Refer to FIG. 13(k)). If the air bubble 10 stagnates, the air bubble 10 may be guided toward the nozzle 4 when ink is ejected from the nozzle 4, resulting in defective ejection at the nozzle 4, thereby producing faulty images.